Marchin Renée M, Medlyn Belinda E, Tjoelker Mark G, Ellsworth David S
Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia.
Glob Chang Biol. 2023 Nov;29(22):6319-6335. doi: 10.1111/gcb.16929. Epub 2023 Sep 12.
High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (A ) and stomatal conductance (g ). However, evidence for decoupling of g from A under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum T > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced A and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced-g was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.
高气温会增加大气水汽压差（VPD）以及干旱的严重程度，对全球森林构成威胁。植物通过调节气孔来最大化碳获取并最小化水分流失，从而使净光合作用（A）与气孔导度（g）紧密耦合。然而，已经发现了在极端高温下g与A解耦的证据。这种反应既提高了叶片在高温事件中的存活率，也迅速耗尽了可用水分。为了了解这种解耦的普遍性和重要性，我们在炎热天气（最高温度T＞40°C）下，对生长在澳大利亚悉尼温室或城市地点的26种乔木和灌木物种的叶片气体交换进行了测量。我们假设，在炎热天气下，水分充足的植物会表现出A降低，并利用蒸腾冷却导致气孔解耦，而水分有限的植物则会依靠其他机制来避免致命温度。相反，无论植物的水分状况如何，都发现了气孔解耦的证据。在热浪期间，水分充足的植物的蒸腾作用比模型预测值高23%，这有效地将叶片温度冷却到气温以下。对于更炎热、干旱的植物，热浪期间蒸腾作用的增加更为明显——g比模型预测值高77%。在城市地点的大多数阔叶常绿和阔叶落叶物种中都发现了气孔解耦现象，包括一些水分有限的枯萎树木。解耦可能仅仅是高温对植物叶片物理影响的被动结果，即通过增加水蒸气的角质层导度，或者气孔解耦可能是一种适应性反应，在高温下由气孔开放主动调节。这种温度响应尚未包含在任何陆地表面模型中，这表明在高温和高VPD条件下，蒸散的模型预测可能被低估。
